Switch to plain StringIO for Py2

SevereOverfl0w · SevereOverfl0w · commit dce101a56968 · 2016-10-12T15:51:15.000+01:00
0 length strings break without this change, apparently you cannot
'write' to a cStringIO.StringI

It also doesn't support unicode.
diff --git a/nrepl/bencode.py b/nrepl/bencode.py
@@ -12,7 +12,7 @@
 
 
 try:
-    from cStringIO import StringIO
+    from StringIO import StringIO
 except ImportError:
     from io import StringIO
 
